Merck Manual

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Rajeev Bhatia

, MD, Phoenix Children's Hospital

Reviewed/Revised Jan 2022
Topic Resources

Bronchiolitis is an acute viral infection of the lower respiratory tract affecting infants < 24 months and is characterized by respiratory distress, wheezing, and/or crackles. Diagnosis is suspected by history, including presentation during a known epidemic; the primary cause, respiratory syncytial virus, can be identified with a rapid assay. Treatment is supportive with oxygen and hydration. Prognosis is generally excellent, but some patients develop apnea or respiratory failure.

Bronchiolitis often occurs in epidemics and mostly in children < 24 months, with a peak incidence between 2 months and 6 months of age. The annual incidence in the first year of life is about 11 cases/100 children. In the temperate northern hemisphere, most cases occur between November and April, with a peak incidence during January and February.

Etiology of Bronchiolitis

Most cases of bronchiolitis are caused by

Pathophysiology of Bronchiolitis

The virus spreads from the upper respiratory tract to the medium and small bronchi and bronchioles, causing epithelial necrosis and initiating an inflammatory response. The developing edema and exudate result in partial obstruction, which is most pronounced on expiration and leads to alveolar air trapping. Complete obstruction and absorption of the trapped air may lead to multiple areas of atelectasis, which can be exacerbated by breathing high inspired oxygen concentrations.

Symptoms and Signs of Bronchiolitis

Typically, an affected infant has symptoms of upper respiratory infection with progressively increasing respiratory distress characterized by tachypnea, retractions, and a wheezy or hacking cough. Young infants (< 2 months) and infants born prematurely may present with recurrent apneic spells followed by resolution of apnea and onset of more typical symptoms and signs of bronchiolitis over 24 to 48 hours. Signs of distress may include circumoral cyanosis, deepening retractions, and audible wheezing. Fever is usually but not always present. Infants initially appear nontoxic and in no distress, despite tachypnea and retractions, but may become increasingly lethargic as the infection progresses. Hypoxemia is the rule in more severely affected infants.

Dehydration may develop from vomiting and decreased oral intake. With fatigue, respirations may become more shallow and ineffective, leading to respiratory acidosis. Auscultation reveals wheezing, prolonged expiration, and, often, fine crackles. Many children have accompanying acute otitis media Otitis Media (Acute) Acute otitis media is a bacterial or viral infection of the middle ear, usually accompanying an upper respiratory infection. Symptoms include otalgia, often with systemic symptoms (eg, fever... read more Otitis Media (Acute) .

Diagnosis of Bronchiolitis

  • Clinical evaluation

  • Pulse oximetry

  • Chest x-ray for more severe cases

  • Respiratory syncytial virus (RSV) antigen test on nasal washings, nasal aspirates, or nasal swabs for seriously ill children

Diagnosis of bronchiolitis is suspected by history, examination, and occurrence of the illness as part of an epidemic. Symptoms similar to bronchiolitis can result from an asthma Wheezing and Asthma in Infants and Young Children Wheezing is a relatively high-pitched whistling noise produced by movement of air through narrowed or compressed small airways. It is common in the first few years of life and is typically caused... read more exacerbation, which is often precipitated by a respiratory viral infection and is more likely in a child > 18 months of age, especially if previous episodes of wheezing and a family history of asthma have been documented. Gastric reflux with aspiration of gastric contents also may cause the clinical picture of bronchiolitis; multiple episodes in an infant may be clues to this diagnosis. Foreign body aspiration occasionally causes wheezing and should be considered if the onset is sudden and not associated with manifestations of upper respiratory infection. Heart failure associated with a left-to-right shunt manifesting at age 2 to 3 months also can be confused with bronchiolitis.

Patients suspected of having bronchiolitis should undergo pulse oximetry to evaluate oxygenation. No further testing is required for mild cases with normal oxygen levels, but in cases of hypoxemia and severe respiratory distress, a chest x-ray supports the diagnosis and typically shows hyperinflated lungs, depressed diaphragm, and prominent hilar markings. Infiltrates resulting from atelectasis and/or RSV pneumonia may be present; RSV pneumonia is relatively common among infants with RSV bronchiolitis.

RSV rapid antigen testing done on nasal washings, nasal aspirates, or nasal swabs is diagnostic but not generally necessary; it may be reserved for patients with illness severe enough to require hospitalization because it may guide isolation and bed assignment decisions. Other laboratory testing is nonspecific and is not routinely indicated; about two thirds of the children have white blood cell counts of 10,000 to 15,000/mcL (10 to 15 × 109/L). Most have 50 to 75% lymphocytes.

Prognosis for Bronchiolitis

Prognosis is excellent. Most children recover in 3 to 5 days without sequelae, although wheezing and cough may continue for 2 to 4 weeks. Mortality is < 0.1% when medical care is adequate. An increased incidence of asthma is suspected in children who have had bronchiolitis in early childhood, but the association is controversial because the children who later develop asthma may be more severely affected by RSV and therefore more likely to come to medical attention. The incidence seems to decrease as children age.

Treatment of Bronchiolitis

  • Supportive therapy

  • Oxygen supplementation as needed

  • IV hydration as needed

Treatment of bronchiolitis is supportive, and most children can be managed at home with hydration and comfort measures.

Indications for hospitalization include

  • Accelerating respiratory distress

  • Ill appearance (eg, cyanosis, lethargy, fatigue)

  • Apnea by history

  • Hypoxemia

  • Inadequate oral intake

Children with underlying disorders such as cardiac disease, immunodeficiency, or bronchopulmonary dysplasia Bronchopulmonary Dysplasia (BPD) Bronchopulmonary dysplasia is chronic lung disease of the neonate that typically is caused by prolonged ventilation and is further defined by degree of prematurity and extent of supplemental... read more Bronchopulmonary Dysplasia (BPD) , which put them at high risk of severe or complicated disease, also should be considered candidates for hospitalization.

In hospitalized children, 30 to 40% oxygen delivered by nasal cannula, tent, or face mask is usually sufficient to maintain oxygen saturation > 90%. Endotracheal intubation Mechanical Ventilation Initial stabilization maneuvers include mild tactile stimulation, head positioning, and suctioning of the mouth and nose followed as needed by Supplemental oxygen Continuous positive airway... read more is indicated for severe recurrent apnea, hypoxemia unresponsive to oxygen therapy, or CO2 retention or if the child cannot clear bronchial secretions. High-flow nasal cannula therapy, continuous positive airway pressure Continuous Positive Airway Pressure (CPAP) Initial stabilization maneuvers include mild tactile stimulation, head positioning, and suctioning of the mouth and nose followed as needed by Supplemental oxygen Continuous positive airway... read more (CPAP) therapy, or both are often used to avoid intubation in patients who are at risk of respiratory failure.

Hydration may be maintained with frequent small feedings of clear liquids. For sicker children, fluids should be given IV initially, and the level of hydration should be monitored by urine output and specific gravity and by serum electrolyte determinations.

There is some evidence that systemic corticosteroids are beneficial when given very early in the course of the illness in children with underlying corticosteroid-responsive conditions (eg, bronchopulmonary dysplasia, asthma), but there is no benefit in previously well infants.

Antibiotics should be withheld unless a secondary bacterial infection (a rare sequela) occurs.

Bronchodilators are not uniformly effective, but a substantial subset of children may respond with short-term improvement. This is particularly true of infants who have wheezed previously. Hospital stays probably are not shortened.

Ribavirin, an antiviral drug active in vitro against respiratory syncytial virus (RSV), influenza, and measles, is probably not effective clinically and is no longer recommended except for immunosuppressed children with severe RSV infection; it also is potentially toxic to hospital staff. RSV immune globulin has been tried but is ineffective.

Two monoclonal antibodies used for RSV prophylaxis in infants and young children are available in the United States. Nirsevimab is preferred but may not be available to some infants; if it is not available, eligible high-risk infants and children should receive palivizumab (see also Prevention of RSV Prevention Respiratory syncytial virus and human metapneumovirus infections cause seasonal lower respiratory tract disease, particularly in infants and young children. Disease may be asymptomatic, mild... read more for indications).

Key Points

  • Bronchiolitis is an acute, viral, lower respiratory tract infection affecting infants < 24 months and is typically caused by respiratory syncytial virus (RSV), rhinovirus, or parainfluenza virus type 3.

  • Edema and exudate in medium and small bronchi and bronchioles cause partial obstruction and air trapping; atelectasis and/or pneumonia cause hypoxemia in more severe cases.

  • Typical manifestations include fever, tachypnea, retractions, wheezing, and cough.

  • Clinical evaluation is usually adequate for diagnosis, but more severely ill children should have pulse oximetry, chest x-ray, and rapid antigen testing for RSV.

  • Indications for hospitalization include accelerating respiratory distress, ill appearance (eg, cyanosis, lethargy, fatigue), apnea by history, hypoxemia, and inadequate oral intake.

  • Treatment is supportive; bronchodilators sometimes relieve symptoms but probably do not shorten hospitalization, and systemic corticosteroids are not indicated in previously well infants with bronchiolitis.

  • Passive immunoprophylaxis against RSV is recommended for all infants.

Drugs Mentioned In This Article

Drug Name Select Trade
Copegus, Moderiba, Rebetol, RibaPak, Ribasphere, Ribasphere RibaPak, RibaTab, Virazole
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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